The valuable part of a tree is the butt area where the felling cut is made and there has been increasing concern over the damage being done to this area by severing devices now in use, particularly shears. To overcome this, numerous proposals have been made adapting chain saws to felling heads in place of the shear blades and a large number of the same are in operation. The main drawback, however, of chain saws is their fragility.
Numerous proposals have also been made adapting circular saw blades and cutting disks for cutting trees and severing standing trees. In the majority of the known devices, the circular saw blade is mounted on a central axle and, in some cases, are rotated at high speeds, i.e. rim speeds in the range of 10,000 to 20,000 f.p.m. Some feel these speeds are necessary to give a high quality cut and perhaps more importantly to build up a reservoir of energy which can be applied to produce a rapid cut. Varying degrees of success have been achieved by the devices now known.
One problem with the majority of the known circular saw devices is that they are bulky. One reason for this is that the rotary cutter running on a central axis must have a diameter at least twice that of the tree to be cut plus space required for the hub and axle. It has been found that a minimum size for the saw is about 21/2 times the diameter of the largest tree to be cut. This ratio also holds true for two cutters abreast when each cuts half of the tree.
Another reason contributing to the bulkiness of circular saws is that, in many cases, they require protection particularly where slim disks are used to minimize the kerf loss. Also, in many instances, the saw blade is retractably mounted in a housing which increases the bulk of the device and requires additional apparatus to effect a reciprocating stroke at least equivalent to the largest diameter of tree to be cut.
Some disks are not protected as they do not retract and while they are less bulky they must be heavy enough to withstand impact loads imposed thereon in addition to those produced by cutting. Also, they run at high rim speeds and thus store a great deal of energy. This permits a very fast cut but the energy release can also produce damaging and dangerous stresses in the disk and its teeth when the disk engages the tree to be cut or, even more seriously, engages stones or bed-rock.
One known device (see Canadian Pat. No. 1,140,029 issued Jan. 25, 1983 to A. Larose) employs two saws in stacked relation to each other with enough space between the saws to support and drive them with a chipping element being disposed between the blades to clear the kerf. The saw is driven at a high speed and not protected and therefore subject to damage. Such arrangement, however, is somewhat more compact in diameter than other known devices because the saws are supported and driven by means between the saw blades thus leaving the upper and lower faces clear, resulting in that the saw diameter need only be somewhat larger than the largest tree to be cut. The penalty in this solution is in the width of the kerf which must be large enough to permit passage of the two saws and the supporting and driving mechanism between them. The kerf is so wide that the wood loss to sawdust is roughly the same as that damaged by shears. This wood can not be used for pulp as it is sheared wood and left behind in the forest. This solution, by some, is felt to be a retrogressive step in the art.
In summary, the known devices for cross-cutting trees, particularly the felling cut, are fragile in relation to the environment where they must work, some of them being dangerously so because of their fly-wheel characteristics. Most of them are excessively bulky, and the centrally driven double saw blade that is not, produces an unacceptable kerf loss.